In battery-driven mobile equipment, low power consumption is required of an individual semiconductor device. Accordingly, as a SDRAM (Synchronous Dynamic Random Access Memory) which is known as one of volatile semiconductor memories, one having as a low-power-consumption mode such as a power-down (PDW) mode, a self-refresh (SRF) mode, or a deep-power-down (DPD) mode has been developed.
There has been conventionally known a technology which automatically shifts the SDRAM to the SRF mode upon detecting a shift of a processor to a sleep state, thereby achieving power savings, while retaining data in the SDRAM (Patent Documents 1 and 2).
Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-140138
Patent Document 2: Japanese Laid-Open Patent Publication No. 2002-358231